Field of the Invention
The present invention relates to the separation of tertiary butyl alcohol (TBA) from ditertiary butyl peroxide (DTBP) by procedures which involve conversion of the TBA to isobutylene and water. In especially preferred practice, the invention relates to the separation of cetane additive quality DTBP from mixtures comprised of TBA, DTBP and also containing hydroperoxide impurities.
Prior Art
Tertiary butyl hydroperoxide (TBHP) is an important chemical of commerce which is generally produced by the molecular oxygen oxidation of isobutane as taught, for example, in U.S. Pat. Nos. 2,845,461 and 5,149,885.
Generally, tertiary butanol is produced in substantial amounts along with the tertiary butyl hydroperoxide during isobutane oxidation. Typically, isobutane oxidate contains 40-45 wt % TBHP and 55-60 wt % TBA. Methods are known for the production of ditertiary butyl peroxide by the catalytic reaction of tertiary butyl hydroperoxide with tertiary butanol and/or isobutylene. See for example, U.S. Pat. Nos. 5,312,998 and 5,288,919 as well as copending applications Ser. No. 08/102,017 filed Aug. 4, 1993 and allowed copending application Ser. No. 08/171,957 filed Dec. 22, 1993.
Generally, the production of ditertiary butyl peroxide by reaction of isobutylene with tertiary butyl hydroperoxide is preferred and indeed where mixtures of tertiary butanol and isobutylene are contacted with tertiary butyl hydroperoxide at reaction conditions, the predominant reaction is between isobutylene and tertiary butyl hydroperoxide and the resulting product is a mixture comprised of tertiary butanol and ditertiary butyl peroxide along with residual TBHP and small amounts of other by-product hydroperoxides and peroxides which are either present in isobutane oxidate or are made during the reaction. Included among such materials are secondary butyl t-butyl peroxide, diisobutylene t-butyl peroxide and the like.
In many applications, e.g. as a diesel fuel cetane improver, the ditertiary butyl peroxide must be substantially free of tertiary butanol and peroxidic impurities and therefore it becomes important to efficiently separate the ditertiary butyl peroxide and tertiary butanol and to obtain a ditertiary butyl peroxide product which is substantially free of tertiary butanol as well as peroxidic impurities.
Numerous studies have addressed the effect of diesel fuel cetane number on heavy-duty diesel engine emissions. The universal conclusion is that cetane additives, which are used to improve diesel fuel ignition quality, reduce the particulate matter, oxides of nitrogen, carbon monoxide, and hydrocarbon emissions. In addition, it is well established that cetane additives provide lower-cost cetane number increase than does the refinery option which involves increasing the natural cetane number through hydrogenation to reduce diesel fuel aromatic content. As the trend toward cleaner burning diesel fuels is growing worldwide, the use of higher cetane fuels is becoming a preferred option for meeting more stringent emission regulations.
Historically, nitrate esters, such as ethylhexyl nitrate, have been used commercially as cetane improvement additives. A review of both the open and patent literature finds numerous references using peroxide based cetane improvement additives. With some of these references dating back to the 1940's, the use of peroxide-based cetane improvement additives in and of itself is not new. Generally, the issues, which prevented commercial acceptance of peroxide-based cetane additives, included manufacturing cost, incompatibility with diesel fuel due to the presence of hydroperoxides, and the low flash point of diesel fuel resulting from the presence of TBA and other light materials which are present in the DTBP used to enhance the diesel fuel cetane number.
The thermal stability of the peroxide additive is one key to determining its compatibility with commercial diesel fuels. Unconverted TBHP and other residue hydroperoxides very easily form free-radicals upon thermal exposure during pre-combustion in the fuel delivery system. These free-radicals also oxidize and destroy diesel fuel lubricity. They must be removed from the DTBP cetane additive in order to provide a commercially acceptable product.
The other key issue is the presence of TBA and other low boiling materials present in the cetane additive. Upon blending of the DTBP cetane additive with diesel fuel, TBA, even present in low levels, will reduce the diesel fuel flash point below pipeline specifications. Thus, the DTBP must be substantially free of TBA and therefore it becomes important to efficiently separate the DTBP and TBA and to obtain a DTBP cetane additive which is substantially free to TBA. Separation by simple distillation is not feasible since TBA and DTBP form a low boiling azeotrope comprised of about 50 wt % of each component.
Recently, it has been established that "cetane additive" quality DTBP, when utilized to increase diesel fuel cetane number, lowers NO.sub.x emission to a greater extent than does the commercially available ethylhexyl nitrate cetane improver (see SAE Publication SP-994 "Diesel Fuel for the Nineties", page 155, Oct. 1993). This will be a significant advantage to the refiner and engine manufacture who are both in the process of developing new diesel fuel and engine technology which can meet the 1998 Clean Air Act target for NO.sub.x reduction from 5.0 to 4.0 grams/Brake Horse Power-Hour. The present invention provides a novel method and integrated process which gives cetane quality DTBP by efficient separation of DTBP substantially free of hydroperoxides and TBA.
The present invention provides a novel method for efficient separation of ditertiary butyl peroxide substantially free of tertiary butanol from mixtures of ditertiary butyl peroxide and tertiary butanol. In addition, during the separation process hydroperoxide contaminants are decomposed so that the recovered DTBP is substantially TBA and hydroperoxide free.